Value to use to fill holes (e.g. 0), alternately a dict/Series/DataFrame of
values specifying which value to use for each index (for a Series) or
column (for a DataFrame). (values not in the dict/Series/DataFrame will not be
filled). This value cannot be a list.

method : {‘backfill’, ‘bfill’, ‘pad’, ‘ffill’, None}, default None

Method to use for filling holes in reindexed Series
pad / ffill: propagate last valid observation forward to next valid
backfill / bfill: use NEXT valid observation to fill gap

axis : {0, 1, ‘index’, ‘columns’}

inplace : boolean, default False

If True, fill in place. Note: this will modify any
other views on this object, (e.g. a no-copy slice for a column in a
DataFrame).

limit : int, default None

If method is specified, this is the maximum number of consecutive
NaN values to forward/backward fill. In other words, if there is
a gap with more than this number of consecutive NaNs, it will only
be partially filled. If method is not specified, this is the
maximum number of entries along the entire axis where NaNs will be
filled.

downcast : dict, default is None

a dict of item->dtype of what to downcast if possible,
or the string ‘infer’ which will try to downcast to an appropriate
equal type (e.g. float64 to int64 if possible)